Method of producing colored coatings of aluminum

ABSTRACT

In procedure for producing colored coatings on aluminum by first anodizing the aluminum surface and then treating such anodically coated surface with alternating current in an acidic bath containing Ni or Co ions to produce a colored deposit in the coating, simplified control with effective coloring results is achieved by providing a content of combined aluminum in the acidic bath while maintaining the bath at a pH, notably about 5 to about 6, which restricts the dissolved A1 ion concentration to a low value.

Uited States atent William Ernest Cooke;

Paul John Sajben, both of Kingston, Ontario, Canada [21] Appl. No. 761,717

[22] Filed Sept. 23,1968

[45] Patented Oct. 26, 1971 [73] Assignee Alcan Research and Development Limited Montreal, Quebec, Canada [72] Inventors [54] METHOD OF PRODUCING COLORED COATINGS SOLUBILITr OF 200- lONS 1N SOLUTION 111 Me" 100 ELECTROLYTE 662,063 4/1963 Canada 741,753 11/1943 Germany OTHER REFERENCES Aluminum, by Richards et al., 1896, p. 95.

Aluminum by Corson, 1926,pp. 162- 163, 165

The Surface Treatment of Aluminum, by Wernicks & Pinner, 3rd ed., 1964, p. 348.

Primary Examiner-John H. Mack Assistant Examiner-R. 1... Andrews Att0rneys Robert S. Dunham, P. E. Henninger, Lester W.

Clark, Gerald W. Griffin, Thomas F. Moran, R. Bradlee Boal and Christopher C Dunham ABSTRACT: In procedure for producing colored coatings on aluminum by first anodizing the aluminum surface and then treating such anodically coated surface with alternating current in an acidic bath containing Ni or Co ions to produce a colored deposit in the coating, simplified control with effective coloring results is achieved by providing a content of combined aluminum in the acidic bath while maintaining the bath at a pH, notably about 5 to about 6, which restricts the dissolved Al ion concentration to a low value.

PATENTEnnm 26 I971 3,616,297

ppm.

SOLUBILITY OF A! 200- IONS IN SOLUTION IN Nb" 100 ELECTROLYTE s's e10 INVENTORS WILLIAM ERNEST COOKE BY PAUL Jorw SAJBEM WSW ATTORNEY METHOD OF PRODUCING COLORED COATINGS OF ALUMINUM BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to procedure for producing inorganically colored coatings on aluminum surfaces, i.e., articles of aluminum, such term including aluminum base alloys and thus generally signifying aluminum that is appropriate for anodic treatment. More particularly, this invention is concerned with colored deposits derived from nickel or cobalt ions and produced in accordance with procedure such as described in U.S. Pat. No. 3,382,160, granted May 7, 1968 to Tahei Asada. One notably effective method of the stated character, for producing suchinorganically colored coatings, involves first anodizing an aluminum article, as in sulfuric acid solution, to form an anodic coating, and then subjecting the anodized article to electrolytic treatment in an acidic bath containing metal ions selected from the group consisting of nickel ions and cobalt ions (Ni**, Co), particularly a bath having a pH substantially above 3, e.g., a pH value of at least about 4.

According to the patent, the described process effectuates a colored deposit in the anodic coating, being understood to be a deposit of the selected metal in chemical combination with oxygen, i.e., a compound of the class of oxide or hydroxide. The result, for example in the case of nickel, is a coating having a color that can be described as bronze, of a shade or tone ranging from a light color to a very dark bronze or brown de pending on selected conditions, the colored coating being one that can be sealed and which has good permanence. With cobalt, the coating may have a range of somewhat bronzelike colors, up to a very dark or indeed nearly black shade. It will be understood that the anodic coating itself is advantageously such as to afford, if desired, other desirable attributes of such coatings, e.g., as in protecting the underlying metal surface from corrosion or other deterioration by weather or the like.

The present improvements are specifically related to the step of electrolytic treatment with alternating current and notably in relation to the composition of the stated metal-containing bath, an important aim of the improved method being to afford a more readily controlled and reproducible operation, achieving superior color or facilitating the attainment of superior color, and indeed, more particularly, permitting the effective and easier attainment of an entire range of shades, including very dark tones, in a highly reliable manner.

2. Description of the Prior Art As stated an effective coloring process, as in accordance with the above-mentioned patent, involves first anodizing the aluminum article, e.g., to produce a suitable oxide coating of the so-called porous type, and then passing alternating current between the article and an electrode, for example an electrode of the selected metal of the bath (i.e., nickel, for a bath containing nickel ions), or in some cases of other suitably conductive material, such as carbon e.g., graphite), while the article and electrode are immersed in an aqueous acidic bath containing Ni or C ions. A typical bath composition, for instance, has comprised an aqueous solution of nickel sulfate (NiSO -7 I-I;Q), boric acid (H BQ and ammonium sulfate,

which is (NI-10 50 the bath being adjusted, as with ammonium hydroxide or sulfuric acid, to have a pH in the range of about 4 to 4.5. Advantageously in accordance with a further invention, distinct from the subject matter of the present improvement but useful with it, the bath may also include magnesium ions, e.g., supplied as magnesium sulfate (MgSO '7 H 0), functioning to reduce the incidence of certain defects, called spalling, and being the occurrence of minute uncolored spots or localities when darker shades of color are sought to be produced. With this alternating current treatment. any selected one of a range of bronze or like colors can be achieved, being understood to be effectuated by deposit in the oxide coating of nickel (or cobalt) material, presumably as the oxide. This colored coating is quite permanent and may be sealed in a conventional manner, e.g., with hot water.

It has heretofore been found that the procedure as described above is somewhat adversely affected by a buildup of aluminum ions in the bath; this has been proven by experience with nickel-containing baths, and tests have demonstrated like adverse effect in the cobalt baths. While the solution can be initially constituted to be essentially free of aluminum ions, it appears that over repeated use of a bath, slight amounts of the anodic film on the work may become dissolved, with corresponding slow accumulation of dissolved aluminum in combined form. Moreover, if pieces of aluminum metal accidentally fall into the bath, as from a work load that consists of a collection of small pieces, or otherwise, such occurrence can also lead to aluminum ion contamination. especially if any aluminum metal, which falls unnoticed into the solution, should rest in contact with a counterelectrode, thereby being subjected to alternating current corrosion.

In consequence effort has been made to avoid accumulation of aluminum ions in baths of the composition exemplified above, e.g., in practical use of the nickel baths, either by care in operation or by taking special steps to purify the bath of aluminum contamination, i.e., to remove dissolved aluminum, or by reconstituting the bath. In particular it was found that if more than about 10 p.p.m. (parts per million), by weight) of aluminum ions (meaning AF) was present in such baths, a defect, that might be called oxide-spalling, may be encountered especially if the process is operated at the higher voltage or longer time needed to achieve a very dark bronze shade; that is to say, minute uncolored spots or areas tend to appear, whether or not the bath includes magnesium sulfate (as mentioned above) to reduce the spalling tendency. Somewhat higher contamination, before spalling occurs, may be permitted when less dark shades are wanted (e.g., 50 p.p.m. for medium tones), but the aluminum-ion effect remains a difficulty. Indeed it is very desirable to be able to use the same bath for the entire ranges of shades or tones, as may be differently required for successive work loads. In consequence procedure for greatly limiting the content of aluminum compounds or for removing aluminum ions, has been recommended and employed.

Investigations have also been conducted relative to the function of the stated bath throughout an entire, wide range of pH valves (outside the heretofore recommended 4to 4.5), utilizing nickel-containing solutions that were free of aluminum ions. These investigations have shown that whereas only very light shades of color can be achieved at low pH values, e.g., below 3.5, it is possible to obtain a very wide variety of shades, from light tones up to an intense and attractive, very dark bronze coloration at a pH in the range of about 4to 4.5. As the pH of the bath, for the alternating current treatment, is raised through values above 4.5, i.e., up to about 6 or so, it is found that deep colors can still be reached, but the product is apt to be characterized by an objectionable grey overtone. While this grey overtone may be tolerated for some purposes of the coatings, it interferes with the desired richness of color, especially in the dark bronze shades. This grey overtone seems most serious at pH of about 5, but appears through the entire range up to about 6; while some lighter color can be produced at higher values of pH that are still acidic, the darker shades are not then attainable.

ment including a transfer of the entire solution, or to reconstitute the bath.

For the improvement of the coioring process (with N i and likewise with Co and particularly for facilitation of its operation and control, it has now been discovered that if instead of avoiding aluminum compounds, a content of combined aluminum is kept in the bath or available in the bath at a significant value, which can be very low or represent a very low minimum, or which may be advantageously be selected at the moderately low amount or minimum of about 10 p.p.m. Al but nevertheless can extend to larger quantities, and if the bath is kept at a suitable pl-l above 4.7, most preferably in the range of about 5 to about 6 (instead of the previously indicated range of 4 to 4.5), the process may be performed with unexpected advantage and full realization of color potentiality. It now appears that under these circumstances, notably where the pH has a value upwards of 4.75, the solubility of aluminum is so reduced in the acidic solution that the actual dissolved aluminum ion concentration is automatically kept at a value of not more than l ppm, and indeed apparently at not more than about 2 to 3 p.p.m. in the preferred pH range of 5 to 6, while all excess of combined aluminum remains in undissolved state. At the same time it has been found that with these conditions and apparently because of the dissolved presence or availability of a small amount of Al ions, the above noted grey overtone is avoided, permitting effective achievement of the desired richness of color of the very dark shades.

A special feature of the 28 invention is that a presently preferred mode of practicing it is of marked simplicity in that ordinarily no special account need be taken of the actual concentration of available, combined aluminum, so long as there is at least enough of it to insure saturation of the bath with dissolved aluminum ions. That is to say, in the pH range between about Sand about 6, the solubility of aluminum is so low that there cannot be enough ions present to interfere with proper realization of the entire color range, yet with the aluminum ions in saturated solution, they are always sufficiently present to obviate the grey overtone heretofore encountered in aluminum ion-free baths. Thus an unusually satisfactory mode of operation is: (a) to make up the bath originally with a deliberate addition of aluminum salt, such as aluminum sulfate, in sufficient amount to provide about 5 p.p.m., or even more, of available aluminum, meaning aluminum in combined form, i.e., combined aluminum which is capable ofoccuring as dissolved Al ions or as undissolved Al compound dependent on the pH of the bath; and (b) thereafter simply to operate the bath at the above stated, preferred pI-l, without further attention to aluminum content except referably to make sure that there is always some undissolved aluminum compound present.

Since in practice under the circumstances last described the effect will be to precipitate some of the combined aluminum content, which may have been originally dissolved at a lower pH of bath makeup, the solution will have a slightly turbid appearance and at the same time will automatically maintain a sufficient residue of dissolved aluminum ions (for instance), about 2 or 3 ppm. at about pH 5.5), to avoid any grey overtone or the like. In fact, if the bath is kept in the optimum pH range, all that is necessary is to keep it mildly turbid with undissolved or precipitated aiuminum compounds (possibly then appearing at least in part as hydroxide), say by simply introducing at the start sufficient soluble aluminum salt to result in such turbidity, in coaction with the necessary pH adjustment. If the combined aluminum content rises as for any of the accidental or other reasons explained above, the increase will be undissolved, harmlessly deepening the turbidity, but the dissolved Al ions will necessarily remain at the efficient low value (below p.p.m.). Moreover, if the solution for any reason becomes clear, such circumstances (unless there has also been a very large addition of aluminum) can be taken as a warning that the pH is probably not in the correct operating range, as for example by having somehow dropped to a lower value, e.g., 4.7 or below.

The present, improved process thus greatly simplifies control of the coloring operation with these baths, requiring a minimum of chemical determination or testing and of chemical adjustment, yet permitting maximum realization of desired results. Further features and aspects of this invention are described hereinbelow, in connection with presently preferred exemplification thereof.

BRIEF DESCRIPTION OF THE DRAWING The accompanying drawing is an illustrative curve approximately representing the solubility of aluminum ions in a nickel-containing acidic bath of the character herein described, and particularly showing the relationship of such solubility to pH in the specially advantageous operating range of the invention.

DETAILED DESCRIPTION In further eludication of the invention, it may be noted that according to tests, satisfactory results of the alternating current coloring treatment, with the nickel bath, have now been found throughout a wide pH range, e.g., from about 4 to about 6, when the total content of available aluminum in combined form, whether undissolved or dissolved depending on pH, is specifically about 10 ppm. Al. With this content of aluminum present, the bath may be utilized to obtain any of the entire range of bronze colors, up to very dark tones, regardless of actual pH in the stated range and without difficulty by reason of grey overtone or the like. While like results occur at a somewhat lower content of combined aluminum, and excellent results are obtainable at an Al ion level of zero to 10 ppm. at pH 4 to 4.5, it nevertheless appears necessary, for preferred operation anywhere in the range extending downward from about pH 4.7, that the total Al content be prevented from rising appreciably above 10 ppm. since in the latter pH range the concentration of dissolved aluminum ions can rise to values ranging from about 40 ppm. (at about pH 4.6) to much higher amounts at lower pH values e.g., above l50 p.p.m. at pH 4.3 and well above 500 p.p.m. as pH 4 is approached.

In the above context, the specific concept of working in the defined pH area of the present invention, i.e., above 4.75, and particularly from about 5 to about 6, and indeed most conveniently from 5.5 to 6, is of unusual value for simplicity of operation. The actual amount of combined aluminum present in the bath need not be controlled to any critical maximum value, but may even rise well or far above 10 ppm, the effect of the stated pI-l being to suppress the dissolved aluminum ions, in fact to keep them at a value usually well below l0 p.p.m. For instance, even though the bath may contain combined aluminum equivalent to as much as ppm. aluminum ions, the actual dissolved aluminum ion concentration at pH 5.5 is not more than about 2 or 3 p.p.m. The undissolved aluminum, in combined form, remains as turbidity in the electrolyte or of course may even in part settle as sediment at the bottom of the tank. At least within reasonable limits such accumulation of undissolved aluminous material does not appear deleterious, yet in the pH range of 5 to 6 the automatic control of dissolved aluminum ions to a concentration of about 3 ppm. or less, and of course below about 10 p.p.m. has a two-fold advantage: there cannot be sufficient dissolved aluminum to cause spelling or otherwise to interfere with attainment of dark shades of color, yet there is enough dissolved aluminum, as now appears necessary, to prevent occurrence of undesirable grey overtone or the like.

As stated, the complete process of producing a colored coating on an aluminum surface of an article, whether sheet or other fabricated piece or structure, involves first anodizing the surface of the work in a conventional manner to produce an anodic oxide coating, for example of the so-called porous type customarily applied for protective or like purposes. While any of a number of known operations may be employed, notably with electrolytes of a group which may be defined as consisting of aqueous solutions of sulfuric acid, chromic acid. or a sulfonic acid such as sulfosalicylic acid, and suitable mixtures of these with other acids or compounds, and while in some cases A.C. anodizing treatment may be feasible, effective results are obtained by anodizing the work with direct current, as for periods of 20 minutes, to 60 minutes, in an aqueous solution of sulfuric acid, e.g., percent acid by weight. The operating conditions of the anodizing step do not appear to be very critical, being selected largely to suit the thickness and other characteristics of anodic coating desired; the requirements of the subsequent coloring step are satisfied over a con siderable range of thicknesses of porous oxide coating on aluminum.

The second step of the process, to which the present improvements are specifically directed, then involves submerging the anodizing aluminum article, if desired after suitable rinsing, in a nickel-containing acidic bath, in which a counterelectrode is likewise submerged, whereupon alternating current is passed through the solution between the article and the counterelectrode, for sufficient time to effect the desired colored deposit in the oxide coating, e.g., a colored oxide or equivalent deposit of nickel. Respecting its principal constituents, the bath may be prepared in appropriate fashion as set forth in the above cited U.S. Pat. No. 3,382,160, of which the disclosure may be deemed to be herein constituted by reference; thus the bath should be a water solution of a suitable nickel salt and an acidic constituent, all in relatively low concentration. The anionic constituents of the bath may be variously selected to provide the desired solubility of nickel ions and suitable acidity of the electrolyte, and supplemental salts or the like may be incorporated, as desired for purposes of stability, buffering or other function which does not interfere with the formation of the colored deposit, and may include salts or the like that result from initial or subsequent pH adjustment. Thus sulfate, borate, acetate and other common anions are appropriate, as will be apparent from the cited patent. A particularly suitable composition for the nickel bath is made up with nickel sulfate and boric acid, each in amounts of the order of l5 to 40 g.p.l. (grams per liter), usually with some ammonium sulfate (say 10 to g.p.l.), and also very preferably, in accordance with another invention as mentioned above, some content of magnesium sulfate, e,g., from 2 to 20 g.p.l. of the conventional hydrated salt.

Addition of a small amount of combined aluminum to the bath in accordance with the present invention may be in the form of any suitable aluminum compound, such as any of a variety of aluminum salts, for instance sulfate, acetate or others, commonly regarded as water soluble e.g., such as used alone would assume a pH in a high solubility range), or conceivably a compound such as aluminum hydroxide, meaning hydroxide recognized as soluble in acidic conditions. A salt of aluminum with an anion that is otherwise present in the bath is generally convenient for the purpose, i.e., sulfate for the baths here exemplified. Indeed choice of an addition most suitable for a given bath can be readily made in the light of known properties of aluminum compounds, the ultimate requirement being simply to have present in the bath a quantity of alurninum in combined form, which will occur as dissolved Al ions or as undissolved Al compound dependent on the pH of the bath. The further special condition of the baths for the present process is that the pH is adjusted to the desired value, e.g., a pH from 5 to 6, with appropriate acidic or alkaline increments as explained elsewhere herein, the actual concentration of dissolved Al ions being then governed, in upper limit, by the specific established pH.

The counterelectrode is made of suitable electrically conductive material as also explained in the cited patent, presently preferred operation being achieved with a graphite electrode, or most advantageously with an electrode of the same metal as in the bath, e.g., metallic nickel for Ni ions or cobalt for Co ions. Operation with baths of this character, in treatment ofa suitably anodized article, involves passage ofalternating current at a conveniently moderate voltage, say from 5 to 20 volts, for a correspondingly appropriate time, e.g.,

from a few minutes up to 10 or 15 minutes, sometimes with increasing voltage or other appropriate electrical control. in general, the shade of color achieved depends on the time of treatment and on electrical conditions; for instance, light shades are produced by relatively brief treatment with lower voltages, whereas very dark bronze colors require a longer time or greater electrical values.

For further explanation, the drawing indicates the manner in which the solubility of aluminum ions in a nickel sulfateboric acid-ammonium sulfate electrolyte of the sort here described (taken at 25 C.) varies with pH in the range from 4 to 6. The solubility (in p.p.m. Al ions) is relatively high as the pH falls to 4.0, and correspondingly decreases as the pH rises to 6, being of the order of 70 ppm. in the region of pH 4.5, and dropping to and below 10 p.p.m. above pH 4.7, e.g., about 4 ppm. at pH 4.75. Of course, in alkaline ranges, i.e., above pH 7, combined aluminum becomes more soluble, e.g., rising from the low point of the illustrated curve, but this characteristic is of no significance in the present invention, which is related to acidic baths.

It will be readily seen that at the higher pH values of the curve, up to 6 or more, the solubility of aluminum ions is drastically suppressed, regardless of the amount of available, combined aluminum that is present in the bath, i.e., as suspended or settled precipitate. The corresponding pH range contemplated by the invention, generally designated A in the drawing, thus affords the advantage of automatic control of dissolved Al ions (meaning, as stated, A1 ions) at a desirable, low value.

By way of specific example, an aqueous bath (balance, water) is made up to have the following initial composition in grams per liter:

NiSO; 7Hg0 25 g.p.l.

MgSo -H,0 20 g.p.l.

H 25 g.p.l. (NH ),SO. l5 g.p.l.

To this bath may be added, in accordance with preferred practice of the invention, an amount of an aluminum compound equivalent to 5 to 10 ppm. Al ions, e.g., 0.06 to 0.12 g.p.l. of aluminum sulfate (AI SO 'I8H O). Thereafter the pH is adjusted (usually upward) to the range of 5 to 6, as for example a target value of 5.75. As a result, no more than a few ppm. of aluminum ions remain in solution, and the excess combined aluminum is precipitated, e.g., as hydroxide, and will impart a somewhat clouded or turbid appearance to the bath. The pH adjustment, and subsequent maintenance of pH in the desired range, can be achieved by additions of ammonium hydroxide or dilute sulfuric acid as necessary.

With the thus constituted and the desired conditions established by standard pH determination, the basic procedure of the above cited patent may then be followed. Aluminum articles to be colored first have their surfaces anodized by conventional anodic treatment with direct current, e.g., for periods upwards of 20 minutes in a 15 percent sulfuric acid solution. Thereafter the anodically coated article, suitably rinsed, is immersed in the stated nickel-ion-containing bath, with a counterelectrode made of nickel sheet, bars, rods, or the like. Upon passage of alternating current, for a selected length of time, a desired surface of the article. For instance, at voltages of 10 to 15 and with treatment times of 5 minutes or more, bronze shades from light to dark are achieved. For a very dark bronze coating the treatment can be continued for an additional 5 minutes at a higher voltage, e.g., 17 volts. These permanently colored oxide coatings can be sealed in a conventional manner, e.g., using water at or near l00 C. Over repeated use of the bath, replenishment of Ni ions may occur by consumption of the nickel electrode if employed, which is then is then replaced at intervals, or additional nickel compound can be added as necessary.

Ordinarily, no special chemical testing or control is needed as to aluminum ion content, over long periods of use of the bath. Even if the aluminum content tends to build up somewhat for reasons explained above (although such tendency may be less at the higher pH range of 5 to 6), any result ing excess of combined aluminum beyond the original to p.p.m. will not remain in solution, but will appear as an increase of turbidity, or as a settled deposit at the bottom of the tank. Of course, the pH of the bath should be checked from time to time, and corrected as necessary, Indeed even then, the process is further simplified in that if the bath becomes clear despite the presence of at least the original 5 to 10 p.p.m. Al, this is an indication that a test of pH will show it has fallen substantially below the preferred minimum of 5.

In summary, the process involves having combined aluminum available in the bath and maintaining the pH at a valve or in a range as indicated, to keep the dissolved Al ion content suitable low. While with operation at pH 5 to 6, the total required Al content may in a broad sense be no more than that which becomes soluble, or alternatively may be sufficient at a level as low as about 2 p.p.m. (it being presently conceived that the concentration of dissolved Al ions then appearing at the selected pH will suffice, whether at or even below such level), a most convenient procedure is to keep the bath at a saturated concentration of dissolved A1 ions, e.g., in general by pH control to maintain such concentration at not more than about 10 p.p.m., or specifically by operating in the pH range of about 5 to 6 which is understood to impose such limitations in the dissolved ion content automatically. Thus with the pH maintained in the last-mentioned range, a notably simple mode of operation is to have enough A1 present to produce visible turbidity. For this purpose, there is special advantage in explicitly providing an amount of aluminum compound equivalent to at least about 5 p.p.m. Al ions, but as explained, a virtue of the process is that much more combined aluminum can be tolerated since the excess is kept undissolved, e.g., precipitated.

While the present process has been chiefly described and exemplified with nickel-containing baths, it is understood to be usefully applicable, in essentially the same way and wholly or partly for similar improvement, in the case of cobalt-containing baths. Thus for instance whereas the addition of a large quantity of aluminum sulfate to baths containing Co ions (cobalt sulfate and boric acid; a graphite counterelectrode being very suitable for the AC. treatment with cobalt) such as in examples 3 and 18 of the above-cited patent, was found detrimental at pH 4 to 4.5, good or improved results were obtained by raising the pH of such A l-contaminated electrolytes to about 5.75, thereby limiting the dissolved Al content to a very low value as hereinabove explained.

As will now be appreciated, the process is unusually effective for its desired purpose, namely the achievement of an alternating current coloring step, of certain types, with a minimum need for chemical or other analytical determinations, and especially, in preferred operation, with an automatic control of the aluminum ions, to keep their dissolved concentration at or below the indicated levels.

it is to be understood that the invention is not limited to the specific operations and compounds hereinabove described, but may be carried out in other ways without departure from its spirit.

We claim:

1. In a method of producing an inorganically colored anodic coating of an aluminum surface of an article wherein said surface has first been anodized to produce an anodic coating thereon and wherein alternating current is passed between said anodized surface and an electrode while both are immersed in an aqueous acidic bath containing metal ions selected from the group consisting of Ni and Co, for producing a colored deposit in the coating, the procedure comprising effecting said alternating current treatment while maintaining said bath at a pH in the range of about 5 to about 6 and while maintaining in said bath a content of dissolved Al ions in a substantially saturated amount, said maintenance of a saturated content of dissolved Al ions comprising: 1) adding to the bath an aluminum compound in an amount to provide a quantity of combined aluminum in the bath which is greater than the 2 p.p.m. Al and which is sufficient to establish said saturated content of dissolved Al ions at said maintained pll-l, whereby said addition of the aluminum compound saturates the bath with dissolved Al ions, and (2) maintaining said quantity of combined aluminum in the bath, greater than 2 p.p.m. Al, while maintaining said pl-l as aforesaid during said alternating current treatment, to keep the bath saturated with dissolved Al ions.

2. A method as defined in claim 1, in which the metal ions selected from said group consists essentially of N i.

3. A method as defined in claim 1, in which the amount of said aluminum compound added to the bafii provides a greater quantity of combined aluminum than is sufficient for said saturated content Al ions in the bath at the maintained pH, so that the addition of the compound provides some undissolved Al compound in the bath, and which includes thereafter maintaining said quantity of combined aluminum sufficient to keep at least some undissolved Al compound present in the bath.

4. A method as defined in claim 3, in which the metal ions selected from said group consist essentially of N i, and which includes maintaining said quantity of combined aluminum in bath sufficient to keep the bath at least slightly turbid with undissolved Al compound.

5. A method as defined in claim 4, in which said bath is maintained at a pH in the range of about 5.5 to about b.

6. In a method of producing an inorganically colored anodic coating on an aluminum surface on an article wherein said surface has first been anodized to produce an anodic coating thereon and wherein alternating current is passed between said anodized surface and an electrode while both are immersed in an aqueous acidic bath containing metal ions selected from the group consisting of Ni and Co, for producing a colored deposit in the coating, the procedure comprising effecting said alternating current treatment while maintaining in said bath a content of dissolved Al ions in a substantially saturated amount, and while maintaining said bath at a pH in the range from above 4.7 to about 6 for limiting said saturated amount of dissolved Al ions to not more than about 10 p.p.m., said maintenance of a saturated content of dissolved Al ions comprising: (1 adding to the bath an aluminum compound in an amount to provide a quantity of combined aluminum in the bath which is in the range of about 5 p.p.m. Al and above and which is sufficient to establish said saturated content of dissolved A 1 ions as said maintained pH, whereby said addition of the aluminum compound saturates the bath with dissolved A1 ions, and (2) maintaining said quantity of combined aluminum in the bath, in the range of about 5 p.p.m. Al and above, while maintaining said pH as aforesaid during said alternating current treatment, to keep the bath saturated with dissolved Al ions.

7. A method as defined in claim 3: in which the metal ions selected from said group consist essentially of Ni; and in which said pH-maintaining step comprises maintaining said bath at a pH in the range of about 5.5 to about 6.

8. A method as defined in claim 6, in which the amount of said aluminum compound added to the bath provides greater quantity of combined aluminum than is sufficient for said saturated content of Al ions in the both at the maintained pH, so that the addition of the compound provides some undissolved Al compound in the bath, and which includes thereafter maintaining said quantity of combined aluminum sufficient to keep at least some undissolved Al compound present in the bath.

9. A method as defined in claim 8, which includes maintaining said quantity of combined aluminum in the bath sufficient to keep the bath at least slightly turbid with undissolved Al compounds.

10. A method as defined in claim 8, in which the metal ions selected from said group consist essentially of Ni ions and in which the said electrode is of nickel.

11. A method as defined in claim 10, in which the bath comprises boric acid and nickel sulfate.

12. A method as defined in claim 8, in which the metal ions selected from said group consist essentially of Ni and said bath is maintained at a pH in the range of about 5 to about 6.

13. A method as defined in claim 12, which includes the step of first anodizing said aluminum surface of the article with direct current between said article as anode and a cathode in an aqueous solution of sulfuric acid to produce said anodic coating, said electrode being of nickel and said bath.

Col. 2,

Col. 3,

EDI-MRI ILFLETCHITPgJII. I I; be ting Officer QM PO-IOSO [10-69) Patent No.

In'ventofls) 3,616 ,297 Dated October 26 1971 WILLIAM ERNEST COOKE and PAUL JOHN SAJBEN It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

line 25 after "million" delete end of parenthesis line 50 after "obtain a" delete "very"; line 70 after "practice" insert --has-- line 9 before "advantageously" delete "be"; line 21 after "2" for "to" read -orline 29 after "the" delete "28"; line 58, after "instance" delete end of parenthesis line 69 for "efficient" read --effective-- line 16 for "anodizing" read -anodized-- line 43 for "20" read 30-- line 49 after "soluble" and before "e. g." insert beginning of parenthesis line 35 "MgSO -H 0" should read --MgSO 7H O-- line 49 after "t e" (first occurrence) insert --so1ution-- line 69, delete "is then" (second occurrence) line 5 after "necessary" change comma to period line 11 for "valve" read --va1ue-- line 13, for "suitable" read -suitably-- line 23 after "5 to" insert --about line 25 for "limitations in" read -limitation on-- line 61 after "coating" for "of" read -on-- line 75, after "than" delete "the";

line 20 after "in" insert -the-- line 24 for "b" read -6- line 26 for "on" (second occurrence) read --ofline 51, for "3" read --6-- line 56 after "provides" insert --aline 58, for "both" read "bath" line 5, after "bath" delete period and insert through which alternating current is passed to said anodized surface comprising boric acid and nickel su1fate.--

Signed and sealed this 9th dam of Mav I972 ROBERT GOTTSCHALK Commissioner of Pa ten ts USCOMNFDC 503764 69 

2. A method as defined in claim 1, in which the metal ions selected from said group consists essentially of Ni .
 3. A method as defined in claim 1, in which the amount of said aluminum compound added to the bath provides a greater quantity of combined aluminum than is sufficient for said saturated content A1 ions in the bath at the maintained pH, so that the addition of the compound provides some undissolved A1 compound in the bath, and which includes thereafter maintaining said quantity of combined aluminum sufficient to keep at least some undissolved A1 compound present in the bath.
 4. A method as defined in claim 3, in which the metal ions selected from said group consist essentially of Ni , and which includes maintaining said quantity of combined aluminum in the bath sufficient to keep the bath at least slightly turbid with undissolved A1 compound.
 5. A method as defined in claim 4, in which said bath is maintained at a pH in the range of about 5.5 to about
 6. 6. In a method of producing an inorganically colored anodic coating on an aluminum surface on an article wherein said surface has first been anodized to produce an anodic coating thereon and wherein alternating current is passed between said anodized surface and an electrode while both are immersed in an aqueous acidic bath containing metal ions selected from the group consisting of Ni and Co , for producing a colored deposit in the coating, the procedure comprising effecting said alternating current treatment while maintaining in said bath a content of dissolved A1 ions in a substantially saturated amount, and while maintaining said bath at a pH in the range from above 4.7 to about 6 for limiting said saturated amount of dissolved A1 ions to not more than about 10 p.p.m., said maintenance of a saturated content of dissolved A1 ions comprising: (1) adding to the bath an aluminum compound in an amount to provide a quantity of combined aluminum in the bath which is in the range of about 5 p.p.m. A1 and above and which is sufficient to establish said saturated content of dissolved A1 ions as said maintained pH, whereby said addition of the aluminum compound saturates the bath with dissolved A1 ions, and (2) maintaining said quantity of combined aluminum in the bath, in the range of about 5 p.p.m. A1 and above, while maintaining said pH as aforesaid during said alternating current treatment, to keep the bath saturated with dissolved A1 ions.
 7. A method as defined in claim 3: in which the metal ions selected from said group consist essentially of Ni ; and in which said pH-maintaining step comprises maintaining said bath at a pH in the range of about 5.5 to about
 6. 8. A method as defined in claim 6, in which the amount of said aluminum compound added to the bath provides greater quantity of combined aluminum than is sufficient for said saturated content of A1 ions in the bath at the maintained pH, so that the addition of the compound provides some undissolved A1 compound in the bath, and which includes thereafter maintaining said quantity of combined aluminum sufficient to keep at least some undissolved A1 compound present in the bath.
 9. A method as defined in claim 8, which includes maintaining said quantity of combined aluminum in the bath sufficient to keep the bath at least slightly turbid with undissolved A1 compounds.
 10. A method as defined in claim 8, in which the metal ions selected from said group consist essentially of Ni ions and in which the said electrode is of nickel.
 11. A method as defined in claim 10, in which the bath comprises boric acid and nickel sulfate.
 12. A method as defined in claim 8, in which the metal ions selected from said group consist essentially of Ni and said bath is maintained at a pH in the range of about 5 to about
 6. 13. A method as defined in claim 12, which includes the step of first anodizing said aluminum surface of the article with direct current between said article as anode and a cathode in an aqueous solution of sulfuric acid to produce said anodic coating, said electrode being of nickel and said bath. 